Exposed Surfaces on Shape‐Controlled Ceria Nanoparticles Revealed through AC‐TEM and Water–Gas Shift Reactivity

Abstract

Aberration-corrected transmission electron microscopy and high-angle annular dark field imaging was used to investigate the surface structures and internal defects of CeO2 nanoparticles (octahedra, rods, and cubes). Further, their catalytic reactivity in the water-gas shift (WGS) reaction and the exposed surface sites by using FTIR spectroscopy were tested. Rods and octahedra expose stable (111) surfaces whereas cubes have primarily (100) facets. Rods also had internal voids and surface steps. The exposed planes are consistent with observed reactivity patterns, and the normalized WGS reactivity of octahedra and rods were similar, but the cubes were more reactive. In situ FTIR spectroscopy showed that rods and octahedra exhibit similar spectra for -OH groups and that carbonates and formates formed upon exposure to CO whereas for cubes clear differences were observed. These results provide definitive information on the nature of the exposed surfaces in these CeO2 nanostructures and their influence on the WGS reactivity.

Effects of Morphology of Cerium Oxide Catalysts for Reverse Water Gas Shift Reaction

“…These ceria nanoshapes exhibited exactly the same trends in activity per surface area for WGS reaction (decreasing from ceria cubes to rods and finally particles) as reported in this study on RWGS reaction (Fig. 5b) [18]. This result is expected based on the principle of the microscopic reversibility [18].…”

“…AC-HRTEM studies providing sub-angstrom resolution [22,24], clearly revealed (111) crystal planes exposed at ceria rods and particles [25,31,62,63], whereas ceria cubes are enclosed by mainly (100) facets [18]. These ceria nanoshapes exhibited exactly the same trends in activity per surface area for WGS reaction (decreasing from ceria cubes to rods and finally particles) as reported in this study on RWGS reaction (Fig.…”

“…Pure and supported ceria nanocubes exhibited twice higher activity in oxidation of hydrogen and ethanol compared to rods and particles [36,37]. Similar trends were reported by our group in WGS and ethylbenzene dehydrogenation [18,38]. In this study we report the effect of well-defined morphologies of CeO 2 on catalytic activity during the RWGS reaction.…”

“…For detailed identification of the exposed crystal facets for the three studied ceria nanoshapes, we refer to work previously reported by our group [18]. AC-HRTEM studies providing sub-angstrom resolution [22,24], clearly revealed (111) crystal planes exposed at ceria rods and particles [25,31,62,63], whereas ceria cubes are enclosed by mainly (100) facets [18].…”

“…Theoretically, oxygen vacancy formation and ability of the catalyst to act as oxygen donor-acceptor is related to the stability of particular crystal facets of the ceria surface [16,17]. Although discrepancy in literature exist regarding exposed crystal planes at the surfaces of ceria rods, recent studies in our group demonstrated that stable (111) crystal planes are mainly exposed at the surface of both ceria rods and octahedra while ceria cubes are enclosed by mainly (100) crystal terminations [18]. Therefore, manipulation of catalyst shape is an approach for tuning the fraction of reactive crystal planes at the ceria surface and this is becoming a novel strategy in obtaining advanced catalytic materials with superior OSC and catalytic reactivity [19,20].…”